


Luminescent

by saltnhalo



Category: Supernatural
Genre: Creature Castiel (Supernatural), Deep Sea Adventures, First Meetings, M/M, Marine Biologist Dean Winchester, Romance, Strangers to Lovers
Language: English
Status: Completed
Published: 2019-06-19
Updated: 2019-06-19
Packaged: 2020-05-14 13:23:35
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 17,222
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19274194
Author URL: https://archiveofourown.org/users/saltnhalo/pseuds/saltnhalo
Summary: Embarking on an expedition to the deepest, most unexplored parts of the ocean is all Dean has wanted to do since he was a kid. Now, his dream is finally coming to fruition, and he can feel in his bones that there is still so much to be discovered.But nothing could have prepared him for what he finds in those uncharted, impossibly deep waters…





	1. Chapter 1

**Author's Note:**

> Wow, finally time to post this! I've had a lot of fun creating this world, and I can't thank [jdragon](http://jdragon122.tumblr.com) enough for the incredible art she created. As soon as I saw it, I knew that it was the one I wanted to claim and write for, and I'm so lucky to have been paired with such a lovely human being and talented artist. j, thank you for your support and hard work and patience, and I'm so proud of this little story that we've come up with together. I hope you like it, and I hope that everyone else likes it too.
> 
> [Check out the art masterpost here!](https://jdragon122.tumblr.com/post/185706271445/deancas-reverse-bang-2019-click-for-higher)
> 
> Thank you also to the DCRB mods, to my beta readers ([adaille](https://archiveofourown.org/users/adaille/pseuds/adaille), [captainhaterade](https://captainhaterade.tumblr.com), [EllenOfOz](https://archiveofourown.org/users/EllenOfOz/pseuds/EllenOfOz) and [rocksalt&honey](https://rocksaltandhoney.tumblr.com/)), and to [pallasperilous](https://pallasperilous.tumblr.com/) and [KreweOfImp](https://archiveofourown.org/users/KreweOfImp/pseuds/KreweOfImp) for title spitballing assistance.
> 
> Enjoy!

Dean Winchester leaves for the depths of the ocean on a Thursday—September 12, 2038, to be exact.

For something he’s been striving towards his whole life, ever since he learned about the ocean and all its mysteries, it’s more than a little surreal for it to finally be happening. Even when he’d been granted his loans, even when he’d started building the ship he’s been designing since he’d started college, it hadn’t quite felt _real_ yet.

It had felt a little more so once he’d sat down with Sam to properly plan out the logistics of his expedition, and now, as he’s onboard his ship and going through his checks, it’s starting to properly sink in.

People have made expeditions into the Challenger Deep before, but only to say _I was here. I did this_. Soldiers, thrillseekers, those intent on reaching the lowest point on Earth for no reason other than bragging rights. Science, in the last twenty years, has turned its eyes to space: to colonizing the moon, to exploring Mars, to reaching out beyond the edges of their galaxy for any sign of intelligent life.

But not Dean. There is so much down there that they don’t know about, entire ecosystems yet to even be _discovered_ , let alone understood, and he’s sure as hell not going to leave the discovery-making to the heavyweight research companies once they turn their attention back to the uncharted depths of the ocean.

“Winchester to control,” he says into the microphone of his headset, settling into the single captain’s chair in front of the ship’s control panel. “Undergoing routine checks prior to departure. Over.” All the preparations he’s having to go through feel like every space movie he ever watched as a kid, but instead of putting on a spacesuit and rocketing up towards the moon, he’s sitting in what is essentially the most high-tech of submarines, and he’s going to be defying gravity in an entirely different way.

Charlie’s voice filters back through his headphones at him: “Control to Winchester, report in when checks are complete, and I swear to god, you better not bust any of my systems while you’re down there or I’ll kick your ass myself. Over.”

Dean snorts—remembering to take his finger off the comms button so that Charlie doesn’t hear. Charlie is just as invested in this ship as he is, having spent just as much time, if not more, working on it. He knows that if he damages any of her tech, he’ll be in trouble—but then again, if he busts anything, it makes it much less likely that he’ll make it back to the surface to get his ass kicked.

So there’s another incentive.

It takes him almost an hour to check everything (and then check it again, just to make sure, because his whole life is kind of riding on things working in more ways than one). All the computer systems are operational, the air filtration and water recycling are good to go, his food and equipment storage are fully stocked, and the pressure distribution mechanism _seems_ to be doing its job. That one he won’t properly find out about until he’s right down in the bottom of the Challenger Deep, so he’s just gotta cross his fingers.

Finally satisfied, he settles back into his captain’s chair and connects through to Charlie. “Winchester to control, checks are complete, all systems appear operational. Over.”

Charlie’s voice crackles back almost immediately. “You’re damn right they do, Winchester. I’ve received your diagnostic data—everything looks good. You’re good to go whenever you’re ready, over.”

They’ve said their goodbyes in person already, tight hugs shared on the deck of the control boat before Dean descended down into his submarine and closed the hatch after himself. All he needs to do is press a button to seal it, and he’ll be able to start his descent. There’s a whole world of scientific discovery waiting down there for him, provided he makes the journey in one piece.

But still—even though he’s been working towards this ever since he first picked his major in marine biology as a fresh-faced college teen so many years ago, it’s a little overwhelming to _finally_ be teetering on the precipice of the expedition he’s been planning for half his life. It’s risky, but science has never evolved through people playing things safe. He needs to do this.

He _wants_ to do this.

“Is Sam there?” he asks Charlie. His little brother has been a blessing on the logistics side of the expedition, and even though he’s not needed for the control center like Charlie is, he’d insisted on coming out on the boat with them to the middle of the Philippine Sea, just to be here for Dean.

He’ll never admit it to Sam’s face, but Dean’s really fucking lucky to have him as a brother.

“I’m here.” It’s no longer Charlie’s voice, but Sam’s, slightly distorted by the quality of the radio. “You ready to go? Over.”

Dean takes a deep breath, staring out through the front window at the sun-dappled water that stretches out in front of him. He’d better enjoy the sunlight while he can, because there won’t be much of it where he’s going. “Just about, yeah,” he says, because this conversation feels too real and too personal now to be punctuating every single exchange with a damn ‘over’. “It kinda doesn’t feel real, you know?”

Across the comms, Sam chuckles—the kind that’s tinged with just a hint of sadness. “I’m not surprised. You’ve been working so hard for this, though, and if anyone can do it, it’s you. Just try not to get eaten by a giant squid or anything, okay?”

“What a way to go, though, Sammy,” Dean quips, grinning to try and mask the nervous energy vibrating beneath his skin right now. “A blaze of glory and a ball of tentacles.”

He faintly hears Charlie mutter something that sounds like, “You’d like that, wouldn’t you,” but before he gets a chance to reply, Sam cuts back in quickly. “ _Gross_ , Charlie, this is not the conversation I wanted to be having with my brother right before he leaves on his dangerous scientific mission.”

“I’m sure Charlie can find you some brain bleach while I’m gone,” Dean says, grinning to himself. Fuck, he’s going to miss these idiots. Even if he can still talk to them while he’s down there, it’s going to be hard not seeing their faces.

Best to distract himself before he starts getting too sappy, or they’ll never let him hear the end of it. “You guys all good on your end?” he asks, diverting all of them back to the task at hand.

“We’re all good,” Charlie says, her voice gaining a serious tone to it once more. “You sure about this, Dean?”

Like he has a choice, at this point. “It’s only my life’s work,” he quips, trying to hide the nervousness that he can feel creeping up on him. “I don’t think saying ‘no’ is really an option any more. The bank would certainly kick my ass.”

Charlie and Sam both chuckle, but he can practically feel their worry radiating through the comms system. “Look, I’ll be fine, okay?” he reassures them. “We designed this ourselves, I’m sure everything will go perfectly. And if it doesn’t, I’ll be back to haunt your asses, okay?”

“I’d like to see you try,” Charlie retorts, while Sam just snorts and says, “Yeah, right.” Their reactions make him feel a little better—ignoring and compartmentalizing is how he’s gotten this far with such an ambitious project, after all—and he powers up the engines determinedly, before he can doubt himself any further.

“Alright,” he says, flicking on all the different systems that he needs to keep himself alive and in contact with his control center. This is serious business now, and requires all of his focus. “I’m green and operational, good to go as soon as the docking ports are retracted on your end. Over.”

There’s a pause on the line, and then the familiar, muted _clunk_ of the ports retracting back into the side of the ship he’s been moored to. His sub takes on a different feeling now that it’s supporting its own weight and floating alone in the water. “Docking ports have been removed,” Charlie says into his headset. “Proceed when ready. Do you want a countdown? Over.”

Dean flexes his fingers around the control sticks and rolls out his neck. This is the moment of truth: the beginning of the expedition he’s wanted to do since college, since high school, since he first learned that the bottom of the ocean was deep and dark and mysterious. And now he’s almost on his way.

“Nah,” he says, grinning to himself, because _holy shit_ , he’s really doing this. “Let’s fucking do this. Over and out.”

And with that, he pushes forward on the sticks and watches as the nose of his ship tips downwards, beginning his long descent into the ocean’s depths.

 

****

Dean quickly realizes that, for all his excitement and anticipation, the descent itself will not be quick.

He’d known how deep he needs to go, after all—almost thirty-six thousand feet to the bottom of the Challenger Deep—and had calculated how long it would take him, but… the actual thing is different. He watches as the first few sunlit feet quickly give way to blue, and then a darker blue, and then a blue even darker than that, until the light has all but disappeared. As he descends, he sees no shortage of animals, and most carry on their way, but some give his ship a quick investigation before they determine that it’s not either food or a predator, and continue on.

As the scientist that he is, Dean notes these encounters down, even though they’re not the ones he’s interested in. It’s still good to have a record of the species he sees as he goes further and further down into the ocean, and he’s itching with curiosity to see what he finds at the very, _very_ bottom.

It takes just over two hours, and even though Dean is not the kind of person who can sit idly for such a long period of time, he sits and watches the depth finder on his control panel tick up higher and higher as he gets closer to the bottom. It’s pitch black now, and the lights on the front of his ship illuminate only dark water, and then fall away at the edge of their reach.

Dean is _alone_ , truly alone, in one of the most inhospitable places on earth.

But he is okay with that, and he continues his descent until, two hours and sixteen minutes after he bid farewell to Sam and Charlie, he reaches the very bottom of the ocean.

He adjusts the controls so that the ship hovers just above the sandy, muddy ocean bed, and looks out over the barren surface. This deep, life forms have to be incredibly hardy to survive, and other expeditions have only ever found little amphipods—and no one has conducted in-depth research on anything found here yet.

Dean is sure that there’s more down here than just tiny crustaceans. He knows it, _feels_ it. There has to be more.

For a moment, he just lets himself sit there—behind the controls of the submarine he’s been designing for years, in the very spot he’s wanted to explore ever since he chose his Zoology major in college, and even before. And now he’s here.

He blows out a long, disbelieving breath, allows himself a giddy grin, and then gets to work.

He sends a quick message to Sam and Charlie to let them know he’s arrived safely, even though he’s sure that they’ll be monitoring his data readings from up on the surface, then starts to check his systems once more. At this depth and with the pressure weighing down on his ship, the smallest thing going wrong could spell disaster for him.

All the diagnostics he runs come back positive. The engines are still fully operational, his hull and all his windows are still intact, and his systems are all running smoothly.

He’s done it. He’s really sitting at the very bottom of the Challenger Deep.

“Holy shit,” he whispers to himself.

 

Dean sleeps, and after he sleeps, he gets to work.

So far, all he’s seen of the seafloor is rather dark and unremarkable. There are no visible plants to speak of, and the surface is muddy and somewhat gelatinous. Dean uses his probe to sample the substance, eyeing it curiously in his tiny lab before properly sealing the sample and storing it away. If it contains any microorganisms, he’ll be able to study them, but otherwise he’ll palm it off to one of his geologist friends.

He moves along the bottom of the ocean, staying close to the seafloor and watching the water illuminated by his lights for a sign of anything moving out there, any living creature that could be down here with him. Apart from the soft, rhythmic beeping of his ship systems, the world down here is silent.

His hours become a cycle of sampling, recording, driving, eating, sleeping. And then, on the fifth day, he finds it.

 

Dean’s been driving along the bottom of the trench for thirty minutes when his lights illuminate a cluster of rocks protruding from the seafloor. They stick out of the mud somewhat, and seem to stretch beyond the reach of his lights. It’s the first sign of any kind of geological variance that he’s seen down here so, his curiosity piqued, he guides the ship closer.

As his lights shine onto the rocks with more intensity the closer he gets to them, he starts to notice tiny glints of blue mineral reflecting back at him from the dark stone.

“What the fuck?” he says, mostly to himself, because it’s not like there’s anyone else down here that he can direct his shock towards. He’s befriended a couple of geologists, and knows a thing or two about rocks, but he’s never seen anything like this, and he _knows_ there aren’t supposed to be formations such as this one quite this deep in the ocean.

And then he drifts closer, and his lights illuminate the dark crevasse in the middle of the rock formation.

A crevasse. In the middle of the Challenger Deep—thought to be the deepest point in the entirety of all the world’s ocean.

“What the _fuck_ ,” he breathes, once more for good measure, because there’s no possible fucking way that this is real.

This isn’t something that he can tackle alone. A quick check of his clock tells him that it’s early evening up on the surface, so he doesn’t hesitate in pressing the comms button to communicate with Sam and Charlie. He’s been chatting with them on and off over the past few days, keeping them updated on what he’s finding, but this is much more urgent. “Pick up, c’mon, pick _up_ ,” he mutters.

“Ground control to Major Dean, what can we do for you?”

“Very funny,” he tells Charlie, rolling his eyes even though he’s thousands of feet below her. “Can you get Sam? This is serious, I—I think I found something big.” Even that feels like an understatement. How is there possibly something _deeper_ than the very bottom of the Mariana Trench, something still _unexplored_ down here?

Charlie sobers up immediately. “Will do,” she says, and then there’s no sound for about a minute. Dean is left alone, staring into the chasm beneath his ship and wondering just what the hell could be down there. His heart is beating double-time in his chest with the possibility of a new discovery at his fingertips— _this_ is what he’d envisioned, _hoped for_ , every step of the way on this journey. Now, to have it actually realized…

“What have you found, Dean?”

It’s Sam’s voice, now, and there’s barely-concealed excitement in his words. They all know that Dean wouldn’t be making a big fuss over nothing—that for him to call them both in, telling them he’s something big, means that he really means it.

He cuts straight to it. “There’s a hole, Sam. A hole in the ground in the Challenger Deep, surrounded by some funky-looking rocks that shouldn’t be here. _None_ of this should be here.”

At the other end, he hears both Sam and Charlie suck in a sharp breath. There are a few moments of silence, and then Sam asks, “Are you going down there?”

“Have you met me?” Dean jokes back, because there’s no kind of challenge quite like that of unexplored territory, of the chance to go where no-one else has ever gone before. Of course he’s going to go in—but joking about it helps to take his mind off whatever could possibly be waiting for him down that hole. Excitement and trepidation swirl in his stomach.

Sam lets out a quiet, somewhat humorless chuckle. “I guess you’re right,” he muses. “Stupid question.” Neither of them say anything for a few seconds, and then Charlie asks, her voice crackling across the tenuous connection:

“Do you think your radio will work down there?”

And that’s the issue.

The radio they designed for the sub only works at a maximum range of forty thousand feet—because it’s not like they were expecting to find an _even deeper_ part of the Trench. They’d assumed that that range would be just fine. But now that Dean may be descending further (how much further, they have no idea), and would be putting his ship into a tight space that could interfere with the signals regardless…

“I don’t know.” Dean’s voice is quiet. Honest. “But you know I need to try. Even if I can’t contact you, you guys have to trust that I’ll be alright. I know what I’m doing.”

There’s another long pause, and then the radio crackles with the static of Sam’s sigh. “We know,” he says, and then the switch flips from worried brother to mission manager. “Send us the exact co-ordinates and run a diagnostic scan of the crevasse and its surrounds. If we don’t hear from you by the time your exploration is meant to be complete, we’ll alert the authorities. Sound like a good deal?”

Trust Sam to be the one to think about this logically. Dean smiles to himself, shakes his head fondly, then presses his comms button once more. “Sounds good to me, Sammy. Don’t miss me too much, okay?”

“Won’t be a problem.” The eyeroll is practically audible, even through thirty-eight thousand feet of water. “Just make sure you come back in one piece, over.”

“I’ll do my best.” It’s a serious promise. The crevasse could be nothing, just an impassable crack in the seafloor, but his gut is telling him that that’s not the case. This could be the finding that he’s been hungering after for so damn long. “I’ll see you in two weeks, guys. Over and out.”

The radio goes quiet. Dean sits in the cockpit of his ship and stares out at the floodlit rocks—at the black, yawning chasm. “What the fuck is down there?” he whispers, and he can feel a pull now, something indescribable settled behind his sternum and in his bones, urging him onwards. _Something_ is down there. He knows it.

“Only one way to find out,” he mutters. He flips the switches for the shields, the sensors, the fine motor control of the ship’s engines, and then slowly, he guides his sub down into the dark unknown.

 

If Dean had thought that his initial descent had been long, this part feels a thousand times more so. The crevasse had given way to a narrow passageway, twisting and turning downwards and just wide enough to permit Dean’s ship. He’s glad that he’d designed it to be spatially economic back when it had only existed in concept, because there’s no way something bulkier would have been able to fit through the dangerously tight rock walls. As it is, he barely manages, guiding the ship through with his manual controls while trying to monitor his cameras and sensors in case he strays a little too close to the walls.

Driving like this is exhausting and nerve-wracking, and by the time he’s ten minutes in, Dean already feels like his shoulders are just a mass of tense, anxious knots. Slowly, slowly, the number shown on the depth gauge rises higher. He’s already five hundred feet deeper, and even though it’s within the range of his radio, he hasn’t been able to contact Sam and Charlie ever since he entered the crevasse.

Instead, his radio had been making weird sounds—crackling, tuning at weird pitches, sounds that had made Dean’s hair stand on end. When the volume got loud enough to make Dean wince at the pain piercing through his head, he’d closed its connection to the sub’s power cell, and it’s been silent ever since.

In a way, it’s almost more eerie.

But still, he continues on, further down into this narrow crevasse in the hopes that he’ll find something new, something no one has even recorded before. After all, there’s no better place to make a discovery than a part of the ocean that shouldn’t, according to all past tests and surveys, even exist.

And then, after what feels like hours of careful driving and tightly-coiled tension, Dean’s sensors tell him that the space beneath him is opening up. The passageway is coming to an end, and he’s finally going to see whatever it is that lies beneath. His heart is beating double-time in his chest, and as he squints down at the darkness below his sub, he starts to see the faintest delicate hints of…

 _Light_.

“You’re fucking joking,” he breathes. “How the fuck…”

He’s seen nothing but pitch darkness surrounding his sub for days, the only light coming from his ship, and now, to be seeing a light source this far under the ocean… It defies every single one of his expectations.

It also proves that he was right—there _is_ something down here, something worthy of note. This will be the discovery that he’s been searching for all these years.

Throwing caution to the wind, he presses his control sticks down further and eases the ship down a little less carefully than he has been so far, because he can see the end in sight. The last few feet of the passage pass by, and then Dean’s ship is surrounded by open water once more, and his jaw drops as he takes in the sight before him.

The passage opens out into a huge underwater cavern—one that Dean would definitely not be able to see, were it not for the thousands of bioluminescent organisms giving the space a soft, ethereal blue glow. Glow worms and luminescent plants adorn the walls and speckle the ceiling of the cavern like tiny underwater stars, and within the cavern itself are hundreds of jellyfish, bright clusters of krill and shrimp, and even a few shoals of tiny, flickering blue fish. The whole of the seafloor glows the same gentle blue color, dotted with seaweed and anemones and interspersed with brine pools. The same blue-flecked rocks that he’d seen at the mouth of the crevasse are what makes up the cave, and for a good minute, Dean can’t help but stare at this magnificent little microcosm of an ecosystem that he’s stumbled upon.

 _Holy_ _shit_.

Dean snaps a picture with the ship’s camera, then sits back against the give of his pilot’s seat and runs his hands through his hair. “What the actual fuck,” he says, for what feels like the twentieth time today, and then he laughs. It’s giddy and joyful and there’s no way he can possibly stop it, because right now he’s _here_ , and he knows that all his years of planning and all the money he’s sunk into his little ship has all paid off.

People are going to lose their fucking minds over this.

For what must be close to thirty minutes, he lets his little craft hover on autopilot at the top of the cavern and simply lets himself _look_. There are new things to be seen wherever he directs his gaze, and he’s never felt so much like a kid in a candy store as he does right now. Where is he even meant to start? He’s already a week into his expedition, but all the data he’s recorded so far absolutely pales in comparison to this. Two weeks is nowhere near enough—he could spend two _years_ down here and he still wouldn’t be satisfied.

But since he doesn’t really want his little brother to call the coast guard on him—not that they’d be able to find him, anyway—he’s stuck with a two week limit.

So he may as well get started.

Dean is loathe to take his eyes off the incredible scene before him, but eventually he forces himself up out of his chair and over to his little lab. He sends out the probes to test for water temperature, salinity, oxygenation, and anything else that might clue him in as to how in the fuck there’s a whole established ecosystem preserved in this cavern forty-thousand feet beneath the surface, then sets up his computer to collect the data. He can’t do much else until he’s fully set up his lab to be able to receive and store his samples, though, so with a command to his computer of, “Verne, play Zeppelin,” he starts to set up his storage tanks and airtight cupboards to the upbeat guitar of ‘Ramble On.’

It takes him an hour or so to complete, but once he’s done, he has a row of small, water-filled tanks and a whole wall of climate-regulated cupboards and drawers ready to receive any samples or specimens he might collect while he’s down here. And with the boring, technical stuff out of the way, it’s finally time to indulge his curiosities and get a better look at what’s going on in this cavern.

First, he maneuvers the ship closer to the rocky wall of the cavern, to the blue-flecked stone spotted with tiny glow worms and faintly glowing algae. They’re arranged in clusters, tiny colonies that look like an entire galaxy laid out before his eyes, and as his ship approaches in proximity, their glow grows stronger and seems to pulse gently; a recognition of something nearby, a warning, a welcoming? Dean could watch them for hours, and he can’t wait to pull his suit out and get a closer look later, but for now, he has the whole rest of the cavern to explore.

He slowly makes his way down the wall, past cracks in the rocks and ledges and all sorts of hidey-holes for different plants, arthropods, even tiny fish that flicker past his ship like ghosts in the water. Dean watches everything with wide-eyed wonder and the knowledge that he is the first person in existence to witness every single one of these species.

Eventually, the rock wall becomes less steep and starts to flatten into a gentle incline, then gives way to the floor of the cavern. The plants become thicker here, the rocky surface giving way to beds of kelp and seagrass and red algae that still manages to have the same glowing-blue quality as every other organism in this place. Tiny fish dart between the plants, and Dean watches in awe as a group of softly glowing jellyfish float by in front of his window.

The shoals of fish evade him as his ship slowly moves across the bottom of the cavern, hovering a few feet over the top of the seagrass beds, but the clusters of tiny krill and shrimp pay him no mind. For a few moments, as one swarm passes by his ship, everything is a brightly glowing blue, and it’s such a stunning sight that Dean finds, once the swarm has passed and he can see out his window again, that he has been holding his breath.

The brine pools are equally as beautiful—holes in the rock filled with luminous blue water, several times saltier than the rest of the water in the cavern and something that Dean can’t wait to investigate further. But the brine pools, while intriguing, aren’t what catches Dean’s eye as his ship drifts further into the center of the cavern.

In the very middle of the cavern floor and right in the heart of the entire structure, there is a dip in the rock. The blue-green kelp grows the thickest here, swaying gently in a current that should not, by rights, exist. And yet.

Dean’s curiosity is piqued.

He eases his ship closer, once again trusting his gut instinct that is telling him _something is down here_. By this point, having been led all the way down to this cavern by it, he’s pretty happy to trust his gut over his head.

Slowly, more and more of the dip comes into view, and Dean sees that he was correct: there’s something glowing down there, a soft blue color that fades in and out in intensity. As he pulls closer, he can start to make out shapes—a long curve, a handful of tendril-looking things, a line of luminescent spots in a pattern.

And then he must get just a little bit _too_ close, because there’s a blur of movement and light and the shapes in the rock crevice are shifting, reassembling, and Dean realizes with wide-eyed terror exactly what he’s looking at.

It’s a creature, so much larger than the size of Dean’s ship, and it’s unlike anything he’s ever seen before.

What had previously been smooth curves and carefully folded wings is now tense and uncoiled and staring straight at Dean through the glass of his ship’s window. It’s long and kind of eel-like, smooth blue skin covered in glowing tendrils and three pairs of massive wings protruding from its body—the scientist in Dean takes a second to ask _wings? Why underwater?_ before the rest of his mind catches up with what he’s seeing. The whole creature is glowing blue, almost white in the spots and markings that decorate its body, and the two round eyes that don’t have pupils but that Dean can _feel_ staring into his soul.

He can’t see teeth, or even a clear mouth, but he also doesn’t know what this fucking thing is or whether it’s dangerous, and he freezes in fear. The creature’s tendrils around its head ripple, as though it’s tense, waiting. Dean knows exactly how it feels. Right now, he’s not sure if he’s closer to being terrified of this unknown, unresearched beast, or excited at the existence of… well, an unknown, unresearched beast.

For what feels like an eternity, they stare at each other through the glass, completely still save for the gentle ripple of the creature’s tendrils.

And then, in what feels like an uncharacteristically human gesture, Dean watches at it tilts his head to the side. All of a sudden, it feels as though it’s regarding him with curiosity, instead of fear and wariness.

Despite the weird-ass situation that Dean has found himself in, he smiles. “Hello, there,” he mutters under his breath. The creature’s eyes twitch—the underwater, gigantic-monster version of a blink, perhaps? “I like the whole glowy thing you’ve got going on,” he continues, because strangely enough, talking to himself is making him feel just a little more confident. “Really fits with the theme of this place. I’m still not entirely sure that I’m not hallucinating, but hey, I’m sure we’ll find out if you decide to eat me.”

The creature tips its head back the other way, and blinks again. Dean watches as the first pair of its wings flexes, and then—it shakes his head.

Dean’s eyes go wide. “You—“ he stammers, lost for words. “You can understand what I’m saying?”

There’s no reply this time, but it blinks.

He tries a different tactic. “Does that mean you’re not going to eat me?”

A pause, and then a slow, deliberate nod of the head.

“Holy fuck,” Dean breathes, feeling as though all the blood has drained from his face. “You… you understand me. You’re like some kind of… fucking underwater dragon thing. Can I call you Toothless?”

The creature’s eyes narrow, and damn Dean’s big fucking mouth. The worst that happens, though, is that it flares its wings just slightly, stirring the water and creating a current that buffets Dean’s ship enough to make him sway where he stands. “Okay, okay,” he says, holding his hands up placatingly. “I’ll keep working on the name thing. Unless… you already have a name, and you feeling like sharing?”

Another blink. Another no answer.

“It was worth a shot, sue me.”

It’s definitely nice to know that he’s not about to be crunched up by a gigantic deep-sea creature, and it sets Dean’s mind a little more at ease—enough that he can stop white-knuckling his control sticks, at least. But then he’s hit by another thought, one that he hadn’t considered a few moments ago but that is now taking up the entirety of his mind and squeezing his chest tight with the realization.

_How the fuck does this animal understand me?_

It feels like he’s followed his nose down the underwater rabbit hole, only here there are no ‘eat me, drink me’ labels, and instead of a talking cat, he’s found… a big, winged, tendril-y monster that somehow understands English. “This is not what I bargained for,” he whispers—honestly, he would have been happy to have found a complex arthropod or a bony fish down here, but this feels way above his pay grade.

Still. He’s a fucking scientist. Even if he can’t quite wrap his head around what he’s seeing here, it’s his damn job to try and understand it, so that’s what he’s going to do. Maybe… not right now, though. “I’m gonna… keep looking around, okay? I’m trusting you not to, like, fuck up my ship or anything. Keep being a big friendly sea-giant, and we’re not going to have any issues.”

The creature’s tentacles ripple, and then it moves its wings and slowly glides closer, circling around the side of Dean’s ship. He feels his body tense up, as though he’s subconsciously bracing for an impact, even though the creature has seemed pretty chill so far. When it disappears out of sight, he bites out a sharp order to Verne to pull up his camera feed onto the front window, watching as it glides past his ship and curls its long body around it. It looks almost curious, the way it examines the technology and metal with fascination, but Dean’s heart rate skyrockets when it reaches out with one of the clawed hands on the end of its wing to touch the smooth side of the sub’s hull.

“Don’t you fucking do it,” he warns, trying to ignore the shake in his voice. He’s forty thousand feet away from any kind of rescue, and completely at this creature’s mercy. It looks strong enough, claws sharp enough, to tear a hole in his ship with just one swipe.

The creature tilts its head again, pauses, and then—

 _Tink_.

There’s a tiny bump to the ship, one that Dean only feels because he’s hyper-aware of everything around him in this very moment—and because he sees, on the screen, one of the creature’s claws poke against the hull.

“You _motherfucker_ ,” he mutters, but there’s a giddy relief sweeping through him, and he can’t help but laugh deliriously when the creature looks up towards the front of the sub, where Dean is standing. It doesn’t have a mouth, but if it did, Dean is almost certain that it would be wearing a shit-eating grin right now.

“Alright, well, I’m off to have my heart attack somewhere else in this cavern and check out some of the other shit in this cave.” It feels weird, talking to the creature while he’s the only one in the ship right now, but it must be able to hear him even through the glass and metal and water. What a phenomenal hunter it must be—Dean thanks whichever poor guardian angel that’s been tasked with looking out for him for the fact that it hadn’t decided to open his ship like a tin can.

Collapsing the camera footage once again still makes Dean nervous, but he sits back down in his pilot’s chair nonetheless and guides his ship forward—slowly, so that he doesn’t freak out the creature. Moments later, it appears off to the side of the sub, just its head visible as it keeps pace with Dean. It’s going to hang around, then—that’s cool, that’s fine, Dean can totally handle being followed by a gigantic winged creature.

Slowly, he explores more and more of the cavern floor, though it’s a little harder to get excited about bioluminescent algae and the presence of shoals of bony fish when there’s a creature swimming alongside him that resembles nothing that has ever been documented before. Still, he tries.

After a while, he gets used to having the creature nearby, and it’s clear that the creature gets more comfortable with Dean, as well. It starts to move more freely, to range further from the ship and then come back, careful as it draws closer not to nudge or buffet the ship. For now, at least, it’s done being cheeky.

They explore the cavern together for a long time, Dean either entranced by the creature or by the number of other small miracles he’s seen today, and the creature entranced by him or (Dean suspects) showing off for him. Eventually, though, the tension and emotions of the last twenty-four hours start to catch up to him, and he finds his energy fading fast. A check of his above-ground clock tells him that it’s just past midday of his sixth day down here, and since he hasn’t slept since his fourth night, the exhaustion of it all is catching up to him in a big way. “You better not eat my ship while I’m asleep,” he mutters as he sets his controls into autopilot and hovers the ship a few feet off the cavern’s rocky floor. It’s not like he actually suspects that the creature will do anything, though—after the last hour or two he’s spent with it following him, he’s pretty comfortable in trusting it.

Dean makes himself a quick meal out of freeze-dried rations along  with a fruit cup and a candy bar, because his stomach is complaining insistently now, and it’s been a big day, so he’s going to let himself splurge a little. Tiredness weighs down his limbs as he tidies up his tiny kitchen, then makes his way towards the stern of his ship to his bunk. Thankfully, when he’d been designing the ship, he’d given himself a decent amount of room to sleep, because space may be important in a craft like his ship, but he also values his fucking sleep, and it was, to him, a fair trade-off.

He had half-expected to lie awake for hours, his mind processing everything that he’s learned and discovered today, but he couldn’t have been more wrong. As soon as his head hits the pillow, Dean is out cold, exhaustion claiming him and pulling him deep down into the realm of sleep.

 

When he wakes, well-rested and refreshed, Dean enjoys a few moments of peace. That drowsy state of in-between is one that Dean relishes, and he hums contentedly to himself now, curling into the warmth of his sheets. What will he find today, he wonders?

And then, like the floodgates opening, everything that had happened yesterday—today? Two days ago? Time is starting to lose its meaning to him—comes rushing back.

He jerks upright in bed.

 _The creature_.

All at once, his heart feels like it’s beating twice as fast, but this time it’s not from fear, but excitement. He’s still here, there are no alarms blaring, which means that it hadn’t chosen to fuck up his ship while he was asleep. No, this heart racing is from excitement—because today is the day he can begin his research in earnest.

Today is the day he starts to figure out just what the _fuck_ is going on down here.

Before he does anything else, he goes to the bathroom and gives himself a quick clean, because even if he’s the only person in the sub for three weeks, there’s no excuse for being gross. Breakfast, however, can wait just a little while—until he’s taken stock of his surroundings and figured out just what he’s going to get up to today.

When he walks into the main control room of the ship, he’s greeted by the sight of the creature’s face through the front window.

It’s lying on the ground in front of the sub, chin resting on its front wings where they’re folded underneath it, the other two sets tucked in against its back. It seems to be waiting, watching, and when it sees Dean, those white eyes widen, and it springs up off the floor.

“Good morning to you too,” he murmurs, amused, as the creature flaps its wings and ripples its tendrils excitedly. “You gonna show me around your home today?”

The creature pauses for a second, and then in a great flap of wings, it’s gone, rocketing up away from the bottom of the cavern. Dean rushes to the very front of his sub and peers up through the curved window to watch it, his breath catching in his throat at the sight—at the beautiful blue creature twirling through the water above. Its wings stretch out, and the bright markings stand out against the darker water of the cavern. Every movement, every twirl and spin and dive, feels as though it radiates pure joy and excitement.

Dean wonders how long it’s been down here, alone.

He hasn’t seen a second one of the creatures, not yet, and if this one is so excited to see him that it had spent god knows how many hours lying in front of his ship and waiting, then… then it might really be the only one here. His heart aches for the lonely creature, even as he smiles at its joyful antics, watches as it swims back to Dean’s ship with a few lazy undulations of its long body.

“I’ll take that as a yes,” he says, amused. The tendrils around the creature’s head ripple, and it wriggles as it settles back down onto the ground by Dean’s sub, much the way an excited cat would.

He can’t wait to find out more about this thing—how it came to be here, how it works, exactly _what_ it is.

But first, breakfast.

As he’s eating, the probes he’d sent out last night return. “Your diagnostics are ready for analysis,” Verne tells him, and Dean scoops up his plate of rehydrated eggs as he makes his way over to his lab area. The computer is already booted up, and Dean mumbles a distracted, “Thanks, Verne,” as he pulls up the information.

What he sees is enough to make him do a double-take.

It should be _impossible_ , what he’s seeing. His depth finder is telling him that he’s further beneath the surface of the ocean than anyone has ever been, and yet…

The temperature is mild. The water is nutrient-rich. And the pressure?

Barely higher than it would be if he were diving at a depth of forty feet, instead of forty thousand.

When he’d packed a diving suit, he hadn’t really thought that he’d be getting any opportunities to use it, before if the data from his probes is correct and he _isn’t_ going to be crushed by the pressure of thousands of feet of water, then maybe he’s going to get a chance after all. There’s only one way to find out.

Dean is quick in feeding himself and getting dressed into the under-layer of his diving suit, since he may as well test his pressure theory and conduct his research all in one hit, then returns to the front of the sub where the creature is still waiting for him. “Ready to get this show on the road?” he asks—and is talking to the creature going to save his insanity, or has he already been down here too long if he’s talking to a winged fish as though it can talk back?

Either way, that’s a problem for later.

The creature flexes its wings as if to say _I’m ready_ , and Dean smiles. “Alrighty, then.” The first thing on his list that he wants to study is possibly the least exciting—because he can’t wait to get his hands on those fish and figure out exactly how they’re surviving down here, let alone the great big winged fuck that’s apparently decided to adopt him—but it’s sure as hell important. After all, the animals down here can’t exist without a food source to feed on, and so the first thing he needs to examine is the algae, the kelp and the seagrass that cover the cave in seas of gently glowing blue.

He might have found some weird shit down here already, but he’s going to do this properly, damn it.

The creature follows his ship all the way up to where the floor of the cavern gives way to rocky walls and up, swimming lazily alongside with half-powered strokes of its wings that only serve to make Dean wonder just how fast it would be able to move, were it not boxed in by the mile-wide cavern. So many questions yet to be answered—and surely some that he hasn’t even asked yet. But he has to stay on task.

He sets the ship onto autopilot when he reaches an outcropping of rock with a good variety of algae and seaweeds, and tucks his basic emergency remote securely into the pocket of his undersuit. Excitement surges inside him as he pulls his diving suit out of storage—testing it and its high-pressure capabilities inside a controlled lab environment was one thing, but it feels like quite another to be preparing to dive down here, and for his first lot of fieldwork as well.

The more protective, pressure-stabilizing oversuit gets pulled on next, along with his gloves, flippers and mask. _I’m really doing this_ , he thinks as he makes his way over to the airlock, flippers in hand. If this preliminary dive goes well, he’ll be able to start collecting samples today, and running his first round of tests to try and figure out what the hell is going on down here.

His heart is beating hard against his chest as he opens the first airlock door and steps through. It closes behind him with a solid-sounding _thunk_ , and then he’s locked in as the chamber begins to fill with water. Dean pulls his flippers on, checks and tightens the straps of his mask, and thanks his best friend for inventing the tech in his suit that will (hopefully) prevent him from freezing or getting crushed to death.

Only one way to find out.

The water reaches his knees, then his waist, then his chest. When it starts to lap at his chin, Dean has to actively fight his nervousness and his panic response, and keep his breathing level. The water creeps up over the mask, and thankfully he can still breathe even as it passes his eyes and eventually covers him completely.

He’s floating now, the chamber filled with water, and when he presses the green button on the wall beside him, the outer door slides open.

He’s now officially floating in the deepest undersea point on earth.

“Holy shit,” he whispers to himself, propelling himself forward with a few kicks until he’s outside his ship and can see the true beauty of the underwater cavern properly for the first time. Everything seems more vibrant, more real, and Dean watches as a small jellyfish floats by, his eyes wide. “Ship’s log,” he says, and the little computer and microphone tucked into his mask beep to life. It’s the best way to take notes when he’s underwater and might not have use of his hands. “First solo expedition outside of the ship. I’m still alive, so the suit must be working. It’s… fucking beautiful down here. I can’t believe it.”

And then he turns on the spot with a leisurely kick of his flippers, and comes face-to-face with the creature.

It’s catching him with those big, bright eyes, looking so much more real and detailed without the thick glass of the ship between them. There are spots of light beneath its eyes, along its wings, towards the end of each tendril, and it truly takes Dean’s breath away when he really, _truly_ looks at it.

“Well, hello there,” he says, his lips curling up into a smile behind the mask. “You’d better not decide that I look tasty now that I’m outside the ship. I have important science and shit to do.”

The creature tilts its head, and then its tendrils ripple, and it rolls over on the spot—playfully, wings flaring out and then tucking back in, curving towards Dean. “Yeah, you’re kinda cute, you know that?” he muses—then remembers that his microphone is still recording. “Better record this for posterity, huh? Not every day that you come face to face with a ‘creature of the deep’, after all.”

He clears his throat, and makes a mental note to cut out all his ramblings before he presents to audio to anyone. “My name is Dean Winchester,” he says, and the creature perks up, its head lifting. “I’m documenting a new species, discovered... September 14, 2038. I’ve never heard of anything like it before. Its defining features are six wing-like appendages, a mane of tendrils, and long body and tail, a humanoid face, and a frightening amount of intelligence.” That’s putting it mildly. The creature curls its tail and ruffles its tendrils again, pleased.

Dean can’t help but grin at it. “What should I call you?” He taps his chin and swims a little closer to the creature, until there’s only a few feet separating them. It’s probably not wise, but whatever. He trusts it. After a few seconds of thinking, the name comes to him.

“Cas,” he declares. “Colossal Abyssal Specimen. Sounds science-y enough. You like it?”

The creature wiggles again, this time in a movement that involves all of its body and tendrils and wings, and seems to radiate pure joy. Dean laughs. “Okay, I’ll take that as a yes.”

The creature— _Cas_ —comes slightly closer, and its first set of wings comes up to curl loosely around Dean. This close, he can see the thinness of their membrane, and the sharp claws that are each almost as long as his arm. “Still trusting that you’re not going to eviscerate me,” he mutters, turning his microphone off for the time being. “You wanna touch me, you’re going to do it nicely, okay?”

Cas pauses, and then nods. Instead of being flared, the claws get curled into an awkward fist, and it looks like the creature is concentrating hard to do so. “Thanks.” Dean can feel himself smiling, wider than he has so far. “Can I… Can I touch you?”

This time, the nod comes without hesitation.

Slowly, Dean reaches out one gloved hand. It’s only a few feet between them, but it feels like a mile, and he can’t help but hold his breath until his fingertips make contact with Cas’s skin, just below its eyes.

All of Cas’s tendrils flare.

There’s a flash of blue light.

And then in the next moment, it is no longer a sea creature floating before Dean, but a human man.

Or, at least, he _looks_ human. His skin is tanned, similar to Dean’s own, but his hair is dark and long enough that it floats around his head like a halo. He’s completely naked, muscled legs kicking languidly in the water, but somehow that’s not the most shocking thing about him.

His eyes are glowing the same faint blue as the sea creature had been, as everything in this cavern does. And he’s not drowning. He’s _breathing_.

For a second, the man looks as shocked as Dean does. And then he smiles, looks down at where Dean’s fingertips are still touching his chest, and mouths ‘hello.’

It’s the last thing Dean sees before everything goes black.

**Notes for the Chapter:**

> [See the gif version here.](https://jdragon122.tumblr.com/post/185706271445/deancas-reverse-bang-2019-click-for-higher)
> 
> J's impression of Cas's viewpoint upon their first meeting!
> 
> [](https://i.imgur.com/jv2wurg)  
> 


	2. Chapter 2

Dean wakes on the floor of his sub.

He knows that it’s the floor because it’s cold and hard against his back, and because there’s a little pool of water surrounding him that has had nowhere to go and so just sits on the steel surface. He can also see his bed from here, when he blinks his eyes open blearily. Why isn’t he there?

 _The man. The man floating underwater._ Breathing _underwater._

Dean sucks in a gasp and sits upright, his eyes wide.

Immediately, there’s a hand on his chest—cautioning, worried. Fingers splay out over his suit, and Dean follows them up, up the tanned arm to where the man is kneeling beside him. He’s still naked, his hair wet and plastered down to his forehead, and there’s a crease of worry between his brows. For a second, it feels as though Dean can’t breathe, his chest constricted—

And then the situation itself sinks in (there’s a strange man on his ship, the same strange man who had been floating without a suit all these miles beneath the ocean) and he scrambles backwards, away from the man and his touch.

“What the—what the _fuck_ ,” he spits out, his whole body tightly-wound and defensive. “Where did you come from? How the fuck—what did you _do_ to me?”

He doesn’t remember anything after the flash of light, after the creature had disappeared and left this impossible man in its wake. All he remembers is waking up in his ship.

The man slowly lowers his hand back down to his thigh, and settles back onto his heels. Withdrawn and passive like that, he doesn’t _seem_ like a threat, but Dean has been thrown for a loop and there’s no telling what could happen. “You—“ he begins, but the single syllable comes out in a rough rasp, the man’s voice hoarse from what must be disuse—or the copious amounts of seawater. How the hell had he been able to breathe, anyway?

He clears his throat and tries again, those piercing blue eyes never once leaving Dean’s. Even in the brightly-lit sub, they still seem to glow with that same ethereal quality. “You were unconscious,” he says, quietly and with a voice so deep that it sends a shiver down Dean’s spine. “I believe I frightened you, and you…” He makes a face, like he’s trying to concentrate. “ _Aghmi ealayh_? _Deficisti_? No… _fainted_ , you fainted.”

Dean isn’t sure what languages he had been speaking the first and second times around, but they definitely hadn’t been English. Still, that’s not exactly high on his list of things to address right now. He’s still not entirely sure that the fainting story is completely true, but when he thinks back…

He _had_ seen a sea-creature become a human right before his very eyes, and _then_ seen the human survive being over seven miles underwater without drowning. If anything is going to shock him enough to make him faint, that would probably do it. Still. He doesn’t trust this impossible man.

“Who _are_ you?”

The man’s lips curl into a tiny smile, and Dean swears that he can see a hint of happiness, of _joy_ , in those blue eyes. “I believe you called me Cas,” he says quietly.

Dean exhales as though the air has been punched out of him. “You’re the creature,” he breathes, the confirmation shocking him to the core. “You were the creature, and now you’re… you’re a _man_. How?”

He has so many questions as to how the fuck he ended up in this situation, face to face with another human (can he actually call Cas that?) in his submarine, at the very bottom of the ocean. There’s nowhere he can really start in unpacking this whole situation, but getting answers to at least a few of his questions seems like a good first few steps.

Cas shifts uneasily at this question, and for the first time, his eyes slide away from Dean’s. He looks down at the floor, fidgets, and then shifts out of his kneeling position so that he’s sitting with his legs crossed. Dean forces himself to avert his gaze.

“ _Bilmiyorum_ ,” he says—then, when Dean’s brow furrows in confusion, repeats himself in English. “I don’t know.”

_I don’t know?_

“How do you not know?” Dean asks incredulously, and Cas gives him an unhappy look.

“I don’t _remember_. All I know is being here, in this cavern. I’ve never known anything else—I’ve never even _tried_ to change my form until you came down here. I knew I could, I suppose, far, far back in my mind, but… I didn’t have a reason to until you appeared.”

Dean sits back, processes that, and blows out a long breath. He doesn’t know where the fuck to start processing this whole situation, and at this point, he’s not sure that he _can_. He may as well go along with the flow.

“Okay,” he says, running his hands through his wet hair, then pressing the heels of his hands against eyes for a second. “Okay.” He drops them into his lap, then raises his eyebrows at Cas. “You’re chill, right? Still not planning to kill me or eat me while I sleep?”

Cas shakes his head solemnly.

“Then we’re all good.”

 

Surprisingly enough, it doesn’t take long for Dean to get used to having a man-slash-creature hanging around all the time. Cas is friendly and curious and, once Dean had dug out a spare pair of pants and shown him how to put them on, really relaxing to be around. Spending so much time on his own was starting to get to him, but now that he has someone to talk to, he’s feeling a lot more settled.

Cas’s knowledge of the cavern and all the organisms inside it also comes in very handy, as Dean quickly finds out.

When Dean starts getting ready for his second cave exploration (hopefully without any faint-worthy surprises this time), Cas insists on coming with him. He’s endlessly curious about Dean and his work and all his technology, and Dean doesn’t see any harm in teaching him about his tech.

“This is what keeps me alive,” he explains, as he pulls his diving suit on over his undersuit. “It regulates the pressure on my body, since we’re so far under the ocean. I don’t totally understand how it works—my friend Charlie made it for me. She also designed this—” He holds up the mask— “which extracts dissolved oxygen from the water so that I can breathe it. Pretty cool, right?”

Cas blinks at him—once, twice—and then reaches for the mask. He turns it over carefully in his hands, inspecting it. “So you can’t breathe underneath the water without this?” he asks, his brow furrowed in concentration.

“Not even a little bit. Human lungs aren’t meant for water, not like a fish’s gills or anything like that. That’s why I need all this fancy crap.” Dean zips his suit up to his neck, tucks his gloves into his belt, and leans back against the wall of the ship while Castiel touches the mask with curious fingers. “I wish I knew how _you_ breathed underwater,” he muses, and Cas looks up at him, head tipped to the side.

“If I figure it out, I’ll tell you,” he says, the tiniest smile curving his lips. Dean finds himself smiling back in response.

He takes the mask back from Cas, and the two of them make their way over to the airlock. Dean had expected Cas to be a little worried about the enclosed space and the sound of the motors working to operate the doors and pumping system, but he simply stands there, unfazed. When he catches Dean looking, he raises an eyebrow, and Dean shakes his head as though to wave it off.

It occurs to him, as he’s securing his mask into place and pulling his flippers onto his feet, that Cas has operated the airlock by himself before. He would have _had_ to, in order to get Dean into the sub.

Whatever he is, he’s clever, that’s for sure. Dean’s curiosity simply mounts more and more with every little thing he finds out about the guy.

They stand side by side as water begins to fill the small chamber, sloshing over their feet and up their legs. Dean keeps an eye on Castiel, watching him to see if he will act any differently to how he had yesterday, if the whole ‘breathing underwater’ thing had been some kind of fluke. He doesn’t want to see the guy drown right in front of him, and he flexes his fingers, ready to slam the emergency drainage button if something does go wrong.

But the water very quickly reaches their chins, then creeps higher, until the tech in Dean’s mask kicks in and starts pulling oxygen out of the water surrounding him. Finally, they’re both fully submerged.

And instead of choking, or gasping, or drowning, Cas is just… floating. Not a care in the world. He survives underwater just as easily as Dean does above the surface, and it’s absolutely fascinating to watch.

And, in fact—the closer he looks…

“Are you even _breathing_?” Dean asks incredulously, staring at Cas’s chest and the way it is most definitely _not_ rising and falling. Cas looks down at himself, then at Dean, and shrugs as though it’s no big deal.

“I suppose not.”

 _What the fuck_.

~

It takes Dean a little while to get over the whole _no breathing_ thing—because every time he looks over at Cas and sees him swimming along in a human-looking body without a care, it fucks his mind up all over again—but the organisms here aren’t going to research themselves. In the end, he has to just shove his minor zoologist freak-out into the back of his mind and deal with the task at hand.

It is kind of hard to concentrate on collecting samples and analyzing the distribution of the algae when Cas is hovering over his shoulder, though. When Dean comes close, the plants glow faintly stronger, but every time Cas comes near, they practically _shine_. Dean makes a verbal note of the reaction, and then has to explain about the tiny microphone tucked into his mask when Cas gives him a confused look.

Distraction or no distraction, the plants he’s examining are so fascinating that soon it’s easy to focus on his work. They move slowly throughout the cavern, Cas watching Dean work with curiosity. Every now and then, he’ll point something out to Dean—an observation of something he’s noticed from his time down here, or a simple, “They don’t like that,” when Dean shines a bright light onto a cluster of algae.

They work for several hours. Whenever they need to move to a new spot, Cas turns back into his creature form—something that scares the absolute crap out of Dean the first time but that he slowly becomes accustomed to. It’s just easier for him to move around, since he doesn’t have flippers like Dean, and it’s not long before he’ll be human-shaped and peering over Dean’s shoulder once again.

He explains things to Cas as he makes notes, pointing out the structure of the plants and theorizing on how they synthesize their food. Anyone who listens to his voice notes is going to think he’s gone crazy, but Dean doesn’t care. Nothing matters as much as the pure interest and curiosity on Cas’s face as he watches Dean work.

Once or twice, Dean catches Cas watching _him_ instead of the plants as he’s explaining something, his eyes glowing that gentle blue and the softest, intrigued expression on his face.

Dean kinda knows how he feels.

 

“What is it like? This… ‘surface’ you speak of?”

Dean looks up from his rehydrated meal to where Cas is sitting by the control desk, looking idly out at the underwater world around them. He should have expected this question at some point, but it’s still so weird, coming from someone so… _human-looking_. It doesn’t make sense, in Dean’s brain, to think about how he’s never seen the surface world. That all he’s known—or all he can remember—is… this.

“Alright, Ariel,” he says, hiding his grin around a mouthful of food when Cas gives him a quizzical look. “Lemme give you the rundown, then. Up on the surface, there are a ton of people who look like… well, me. And you, when you don’t have six wings and a long-ass body. There are almost nine billion of us—crazy, right?—and we live all over the earth’s landmasses.” It’s so hard to figure out what the hell to tell a creature that can only remember this single underwater cavern. What is important? Where does he even begin to tell Cas about everything that the surface world is?

“There are trees,” he settles on. “Big tall trees that allow us to live. And there’s a ton of different species up there with us, over a billion more than are in this cavern alone. They breathe air, not water, and the clouds give us rain, and the sky is—”

“Blue.”

Dean sucks in a sharp breath.

“Blue,” Cas says again, slowly and carefully. He meets Dean’s gaze. “The sky is blue.”

 

Bit by bit, conversation by conversation, Dean unearths Cas’s little knowledges. The taste of certain foods, the sound made by a piano, the orbit of the moon. Cas knows about history, about some of the animals or the plants that Dean had learned about in school, but not about planes or movies. He knows words from more languages than Dean even knows the existence of, but when Dean asks him about them...

"Can you say 'A long time ago, in a galaxy far far away' in Chinese? What about Spanish?"

"Dean, I've told you, I can't conjure up the words. Sometimes they just... come. It's as though your language is just one of many that I know, but I... I don't know how they got there."

"...Hey, Cas, it's okay. No stress. If you don't even get my movie references, though, we've got bigger problems to worry about. Verne, pull up Star Wars: The Force Awakens."

Poking too much into his knowledge can upset Cas sometimes, and Dean always works to fix it, even though his scientist's curiosity insists that he find out more. One night, he has an entire conversation with Cas about the concept of evolution. Another morning, it's ancient building design, or the ice caps, or the stars that shine far above the surface of the ocean. Slowly, Dean starts to figure out the limits of Cas's knowledge—not that he thinks he'll ever be truly successful, or ever understand  _how_ Cas came to know these things, or why he's down here, or why he can't remember  _how_ he ended up here.

Instead, he tries not to focus on the big picture, and instead pinpoints all the tiny things that Cas shouldn’t know—but does—that scramble Dean’s brain even more every time they come across one.

One thing Cas _doesn’t_ know anything about, however, is modern technology. Things in 2038 are a little different to the Earth that Cas seems to know, whatever that is.

The way into the airlock, when Cas had returned Dean to the ship that day when they had first properly met face to face, is simple enough. A big red button on the side of the wall, and the ship had done the rest for Cas. Operating the microwave, though? It’s a whole new ballgame.

He’s learned not to trust Cas with making their dinner, because no matter how many time Dean shows him how to work the buttons, it never comes out right. How the meals always manage to come out either stone cold or half-burnt is an absolute mystery to him.

One thing Cas does get the hang of, however, is talking to Verne.

He finds out quickly that the AI holds an expansive amount of information, and that all he has to do is talk to it. The two of them converse while Dean is working (sometimes in languages he doesn’t even recognize), or Cas will ask for clarification on a reference Dean has made to the surface world (niche) or popular culture (even more niche). He asks Verne about names—the AI’s is a byproduct of Dean’s geekery, Dean’s means either valley or justice, and ‘Cas’ is short for Colossal Abyssal Specimen, but can also be the shortened version of so many other names.

His curiosity is endearing even when he starts to use Dean’s pop culture references against him. Twenty thousand leagues under the sea, and he somehow managed to find Cas.

 

By now, they have a routine. Cas sleeps (if he really does sleep, Dean has his doubts) outside the sub, curled up in the sand in his winged form. Dean eats breakfast alone, then collects samples and data in the morning, suited up and with Cas by his side—sometimes as a creature, sometimes as a man. Dean has gotten used to him shifting by now (has _had_ to, there’s not really any other option) and they simply enjoy each others’ company. Dean is always hyper-aware of where Cas is, especially since he has no concept of personal space, and is more than happy to crowd up next to Dean if they’re looking at something that piques his interest.

(Dean does not lie awake at night, thinking of the touch of Cas’s hands against his suit, how they would feel against his bare skin. He _doesn’t_.)

They head back into the sub to eat lunch, with Cas trying little bites of Dean’s rehydrated meals out of curiosity even though, as they’d quickly discovered, he doesn’t _need_ to eat. Sometimes they keep exploring and collecting samples, but sometimes they just spend the afternoon inside the sub, with Dean studying his samples and cataloguing his data while Cas observes.

Tonight, they’re taking a little break from the routine. Dean has spent so many days doing nothing but research, and he’s halfway through his time down here. There’s less than a week left until he has to return to the surface to meet up with Sam and Charlie again, so he wants to take some time to just… appreciate where he is, and what he has, in this moment.

Which is why he’s geared up in his diving suit for the second time today, and is just floating in the water, fifty feet above where his ship is parked on the floor of the cavern. Down here, time doesn’t really exist—there’s no sunlight, only the everpresent glow of the algae and seagrass and the animals that Dean has been studying, day in, day out. Still, he likes to think of it as nighttime, as it being _special_.

Cas is floating in the water beside him, the two of them looking out at the vast cavern. Dean thinks of old cars and lookout spots, city lights glimmering before them and the heater cranked all the way up.

“It’s beautiful,” he says, mostly to himself. Cas turns his head at the sound, and his lips curl up into a soft, happy smile. He nods once, as if to say _yes, yes it is_ , and Dean can’t help but smile back, mesmerized by those lips.

 _Cas_ is beautiful.

It’s something that Dean can’t say out loud (not now, maybe not ever, considering how little time they have left), but he certainly thinks it. Cas may not be human, whatever he is, but even with his glowy, too-blue eyes, he really is one of the most attractive men Dean has ever seen. Hell, it might even be _because_ of the eyes, although the perpetual bedhead and the refusal to wear anything but pants may help just a little.

Cas raises an eyebrow, and Dean realizes that he’s been staring. He clears his throat and kicks his feet just a little so that he can keep level in the water with Cas, but more to give himself a distraction than anything. “You’ve been human for all of today,” he points out. “Does it bother you? Not taking your… ‘sea creature’ form?”

Cas blinks, then lifts his shoulder in a shrug. _Not really,_ is the clear answer, but in between one moment and the next, he shifts, and suddenly there’s a tendriled creature occupying the space next to Dean instead of the semi-clothed man who had been there before. It’s a testament to Dean’s acclimatization that he barely even flinches, even when Cas’s tail curls loosely around him and flicks him across the chest.

“Yeah, yeah,” he teases, “you’re a flirt. Showing off now, are we?”

It’s so much easier to tease and joke around with Cas when he doesn’t have to meet that piercing, _human_ gaze. Instead, Dean can look into those wide, bright eyes, and interpret the movement of Cas’s wings and his body, as communication.

The tendrils around Cas’s face ripple, and he wiggles slightly, the way a cat would if it were smugly proud of itself. Dean can’t help his fond smile. “Got you,” he teases with a grin. “Go on, then. Go stretch those wings, really show off.”

Cas blinks at him, in that not-quite-blinking way, and then he stretches his wings and takes off. The water is churned by the force of his wingstroke, and Dean finds himself buffeted back a foot or two, but he hardly notices because he’s so focused on Cas.

Those powerful wings propel him all the way across the cavern like it’s nothing, all three pairs stretching out effortlessly, his tendrils tucked in close and slicked back by his speed. He’s like a giant, blue rocket that swoops around in an arc as he reaches the other side of the cavern, then shoots up towards the roof, rock dotted with tiny spots of light. Against the semi-darkness, Cas’s glow and his markings are all the more impressive, and Dean doesn’t bother biting back the small gasp of amazement that he breathes at the sight.

It takes a few moments to realize that, slowly but gradually, all the lights in the cavern are dimming.

The seaweed, the krill, even the jellyfish. The glow that has been present ever since Dean steered his sub down into this cavern is dwindling into near-darkness, and somehow, Cas is shining even brighter. It’s as though he is his own blue sun, bright and powerful and so _breathtakingly beautiful_ that Dean can’t stop staring.

Cas knows it, as well—he flares his wings, swims in complicated patterns that leave bright spots dancing in Dean’s vision, shows off his speed and dexterity. In the cavern’s sudden darkness, he stands out impossibly more than he had before. The display goes on for a while, and Dean can’t be sure how much of it is just Cas stretching his muscles and how much of it is for any possible number of different reasons, but the zoologist part of him aches to know.

The part of him that has spent the last week with Cas, getting to know him and all his idiosyncrasies and slowly falling for this captivating creature, wants to know even more.

Cas comes swimming back towards Dean with a few powerful strokes of his blue-patterned wings, and comes up short just in front of him in a sharp stop. For a moment, they just look at each other, and then Cas shifts back to his human form. His eyes glow in the darkness, and as the light from the rest of the cavern slowly returns, it sends his face into sharp relief.

His expression is unreadable, dark gaze framed by lowered lashes, his lips parted. Dean can’t tell what he’s thinking, and he’s never wished so strongly that he _could_.

They don’t speak of Cas’s display after that, but it’s always there, in the back of Dean’s mind. Lingering.

 

_Cas is not human._

It feels like this is something Dean has to remind himself of every damn day, because he _seems_ so human. He shows emotions, voices his thoughts, gets grumpy and happy and curious just like any other human would. Just… with the added bonus of turning into a sea creature at least five times Dean’s size.

Sometimes it’s really fucking hard to keep in mind, though.

Dean’s clock tells him that it’s past midnight and nearing one in the morning. He’s been hunched over the desk in his lab for hours now, entering his data into his computer and methodically cataloguing every single physical trait he can see in his specimens while still keeping them alive and intact. It’s painstaking work, but he’s almost done. Just another half an hour to go.

It’s only about five minutes later that he realizes Cas hasn’t asked him a question in the last half hour. When he turns in his chair, Cas is asleep on Dean’s bed, sprawled out over the covers with his eyes closed. Totally dead to the world.

While he’s off-guard like this, peaceful and resting, Dean gets a chance to just… look.

His hair, messy and salt-stiffened, falls half-over his eyes, and his arms are pillowed beneath his cheek. In the ship’s low, nighttime lighting, Dean can _just_ see the outlined muscles of his back, the curve and dip that gives way to low-riding pants. He’s beautiful, _so_ fucking beautiful, and Dean can’t help but watch him and think.

He’s only known Cas for a week now, but they get along so well. Cas is curious and fun and even his occasional grumpiness is endearing, and Dean hasn’t had so much damn fun with someone he’s only known a short while since he met Charlie back in college. This, though…

This feels like something else.

And that thought is fucking terrifying, because it _has_ only been a week, and what if it’s only because Dean is stuck down here with no one else to talk to, and Cas isn’t even fucking _human_ , for fuck’s sake.

Dean cuts that train of thought off before it can run away with him and leans back in his chair, staring up at the ceiling.

When he’d departed on the expedition he’s been planning for what feels like his entire life, he hadn’t, even in his wildest dreams, expected _this_. And he has absolutely no idea what to do about it.

But for now, what he _can_ do is finish cataloguing his specimens. He forces himself to sit up with a long-suffering groan—then quickly checks over his shoulder that he hasn’t woken Cas up. He’s still fast asleep.

Dean tears his eyes away again, runs his hands through his hair, and gets back to work.

 

With two days until Dean is supposed to be leaving, the food rehydrator breaks.

It’s only something that needs a minor repair, hopefully, but Dean is feeling the pressure of his remaining time now and repairing what is essentially a glorified microwave is not how he wanted to be spending it. Grumbling under his breath, he drags his tools out from the storage locker and starts to work on pulling it apart, trying to figure out where the problem is.

The sound of the airlock door sliding open signals Cas’s arrival—they had started off with Dean meeting Cas outside every morning, but now the line has blurred, and Cas is more than comfortable letting himself in and out of the sub.

Dean hadn’t realized how comfortable Cas has become, in general, until there’s breath ghosting against the back of his neck, and the radiating warmth of a body behind him.

“Hello, Dean.”

Dean barely manages to keep himself from jumping a foot in the air, but he does tighten his grip on his screwdriver and force his breathing to steady. Suddenly, his heart feels like it’s jackhammering against his ribs.

“Jesus, Cas, give a guy some warning,” he mutters, half-turning to look at Cas over his shoulder. He’s even closer than Dean had expected, and he swallows at the sight of those blue eyes, and Cas’s slightly parted lips, and the way his long eyelashes send sweeping shadows over his cheekbones when he blinks. “Personal space, man.”

“I apologize,” Cas says, although the slight uptick to the corner of his mouth indicates that he’s not quite as remorseful as his words might suggest. His gaze drops, not to Dean’s lips but to his chest, and Dean suddenly remembers how easily Cas had been able to hear him, that first time they met, even from inside the sub.

He tries to calm the beating of his unruly heart, because he’s almost certain that Cas can hear it right now.

Time to change the subject, and quickly, before Dean gets too deeply embroiled in what is starting to feel like a real life rendition of _The Shape of Water_. “So you’re just inviting yourself in now, huh? That’s where we’re at?”

Cas hums, and his gaze lifts once more—but this time, Dean could swear that it pauses at his lips, before meeting Dean’s eyes again. “Yes, Dean,” he says simply, a cheeky sparkle in his eyes. “That is where we’re at.”

 

The day before Dean leaves is the day that they break.

With every day that passes, and every day that brings them closer to his departure, Dean spends less time doing actual science and more time… well, just hanging out with Cas. Today, they’re floating just above the seagrass beds, lying there side by side and looking up at the glow-worm-speckled ceiling. Cas has borrowed a spare pair of flippers so that he can keep up with Dean in his human form (which he does so easily now) and by this point, it’s not weird to look over and see him not breathing, face clear of the mask that Dean needs to survive. It’s just… normal.

It’s _Cas_.

It’s starting to hit him just how little time they really have left.

A jellyfish floats by overhead, alone and adrift in the dark waters, lighting its way with its own gentle glow. Dean only watches it for a moment before he turns to sneak a glance at Cas. His eyes follow its erratic movements, a sedate smile curling his lips.

Dean came down here to find new species, a new world, but it’s dawning on him now that he may have found something much greater.

Later, as they're standing in the draining airlock side by side, Dean is reminded of his thoughts.

“What’s it like?” he asks, a spur of the moment question. “Being down here by yourself?”

The corners of Cas’s mouth curl downwards. It’s a long few moments before he speaks. “Lonely,” he says, “and quiet. I have the other animals, of course, but they don’t say much of note. They are too interested in eating and having sex.”

“Sounds like a good life,” Dean muses, to distract himself from the fact that _Cas seems to know what the other animals are saying?_ He’s constantly learning new things about Cas, and they never cease to amaze him. He sobers quickly, though, as he fully processes Cas’s words. “So there really is just one of you, huh?”

“I suppose so.” Cas is carefully still, until the doors of the airlock slide open and they step into the ship's interior. “I feel like once, there were more, but… I also feel like there’s so much I don’t remember. I probably never will. I didn't even realize that I had the capacity to change my form and possibly leave this cave until... until I met you.”

Dean’s heart aches at the sadness and the pure longing in Cas’s voice. Unable to help himself, he reaches over and takes Cas’s hand, giving it a quick squeeze. “Hey, it doesn’t matter, right? We’re making new memories now. You’ll get to remember that time a weird human came down into your cavern and fucked around in his big metal bean for two weeks.”

It’s not until he’s said it that he realizes the meaning behind his words. Cas _will_ remember—because Dean will leave, he leaves _tomorrow_ , and then they’ll be nothing but a slowly-fading specter to each other, destined to be nothing but a memory.

It hits him like a blow to the chest, and he stops in his tracks. Cas pauses too, watching him with that desperately sad gaze, his wet hair sticking up in all directions and his eyes as blue as the glow of the cavern itself. God, he’s so _beautiful_. Dean is such a moron.

“Cas, I—“ he says, but his throat closes around the words. He doesn’t know what to say, _how_ to say it. The emotion is in there, coiled tightly around his heart and squeezing it tighter with every breath, but his tongue won’t cooperate.

So instead of trying to tell Cas with his words, he does it another way entirely.

He steps closer to Cas, into his personal space, close enough that their breaths mingle. 

"Dean?" Cas asks, barely more than a whisper. Dean can feel the single syllable on his lips, reverberating through his soul.

He reaches out, his fingers brushing Castiel's bare chest gently, then slowly sliding up to rest on the back of his neck. This might be Dean’s one chance, and if he doesn’t take it, he’s going to spend the rest of his life not knowing.

As a scientist, he really fucking hates not knowing things.

And that’s why he closes the gap between them and gently, carefully, presses his lips against Cas’s.

Cas gasps, and Dean braces himself to be rejected, for Cas to pull away, but—after a moment, he melts into it. It feels like a dream when Cas reaches for him, resting his hands lightly on Dean's hips and pulling him in closer. _He’s kissing Cas_. And Cas is kissing him back.

Holy fucking shit.

They kiss until Dean is drunk on Cas’s lips, until his lungs burn for oxygen he has to break the kiss, even though he would happily kiss Cas for all eternity if he could. As they separate, slowly and reluctantly, he meet's Cas's gaze. He’s looking at Dean with an unreadable expression, so many emotions all mixed up that Dean can’t even begin to figure out what Cas is thinking.

Outside the ship, the whole cave is glowing now, brighter than Dean has ever seen it, but Cas doesn’t seem to notice. He only has eyes for Dean.

Cas doesn't speak, and Dean doesn’t even know what to say, where to start. So instead, he lets his gestures speak for him. He takes Cas’s hand, skin against the fabric of his gloves reaching out for him and kissing him like he’s a drowning man, begging for oxygen.

~

They barely make it to Dean’s sleeping quarters before they’re reaching for each other, pulling at soaked clothes and the tight cling of Dean’s diving suit. Cas’s hands are brand-hot, lips trailing lines of fire over Dean’s skin. He topples backwards onto the bed, dragging Cas with him.

Cas is a quick learner.

“Please, Cas,” Dean breathes, _begs_ , his lips pressed against skin and hands grasping desperately. It isn’t long before he tips over the edge, gone in every possible sense of the word.

When Cas comes, the lightbulb of the reading lamp beside Dean’s bed shatters, and they can faintly hear the sound of breaking glass elsewhere in the ship.

After, they curl up together beneath the sheets of Dean’s bed, sweat-damp and with racing hearts. They do not speak.

 

The clock ticks over to midnight, and Dean is lying in his bed, his head pillowed on Cas’s chest.

It’s quiet—their ragged breaths have long since evened out, racing heartbeats calmed to a steady thump. Cas’s fingers card gently through Dean’s salt-stiffened hair, his other arm draped securely over Dean’s shoulders. Dean hasn’t been held like this in a long time, and he’s missed it.

He can feel the need to sleep pulling at him, settling deep into his bones, but he doesn’t want to succumb. Not yet. Giving in means that he’s letting his last night with Cas pass him by—he’s _accepting_ it, letting it happen.

He can’t do that. As inevitable as he knows it is… he can’t.

It’s a long time before either of them speak.

“Dean.” Cas’s voice is barely more than a murmur, carried in the still air and resonating through to Dean’s bones. “I…”

There’s no way to put this into words. Dean gets it.

“I know,” he says quietly. He splays his fingers over Cas’s skin, listens to the not-quite-human sound of the heartbeat beneath his ear. “I… I don’t know what to do.”

For a long time, Cas is silent. All Dean can hear is his steady breathing and the slow _thud_ of his heart. Finally, his breath hitches, a tiny inhale before he speaks.

“What I do know,” he says, each word quiet and measured. “Is that this cavern is my home. It has been my home for as long as I can remember. But you…” When he inhales, his breath is shaking. “Without you here, I don’t know that it would feel as much like a home.”

Dean sits up. In the low lighting of the sub, dimmed for an ambient effect, he finds Castiel’s gaze and holds it. “You mean…” he breathes, but he still can’t say it—to do so means putting himself too far out, making himself too vulnerable. This could backfire on him in the most spectacular of ways.

But Cas just smiles, and inclines his head in a small nod. “Yes, Dean,” he says, and it comforting to know that Cas can’t say it either, that they’re both in the same boat of _holy fuck this is terrifying, but I know that I want it more than anything I’ve ever wanted before_.

Dean bends his head and kisses Cas—slow and soft and holding every thought and emotion that he can’t put into words. They fit together perfectly, skin against skin, curled up beneath the blankets that Dean had never ever intended to share with anyone.

“ _Olani hoath ol_ ,” Cas whispers against Dean’s lips, when they break for a breath. The words feel as though they resonate through to his very soul.

“What does that mean?” Dean whispers, lifting a hand to brush through Cas’s hair. His eyes glow in the low light.

Cas doesn’t answer the question; instead he pulls Dean back down into another kiss with gentle hands and even gentler lips, and Dean is lost to the pull of Cas’s touch and, eventually, the serene embrace of sleep.

~

The next morning, it feels as though there is some unspoken agreement that lingers in the air between them like a tangible thing. They wake tangled together in Dean’s bed, pressed so closely that it’s impossible to tell where one begins and the other ends, and even though Dean has made incredible discoveries on this trip, it kind of pales in comparison to what he’s discovered here with Cas.

Cas is already awake, watching him with gently-glowing eyes. Dean presses a smiling kiss to his lips. “Morning,” he murmurs, and watches the way that the corners of Cas’s eyes crinkle happily.

He wonders what Cas will think of _real_ mornings. Of the sunrise.

They dress, wash and eat together, moving like one single unit after two weeks of being in each other’s space, now so in-tune. Making one-and-a-bit meals every morning and sitting down with Cas, eating and gazing out at the blue-lit cavern through the ship’s windows… it’s come to feel like his new normal. It’s beautiful, and even though Dean is looking forward to returning to his home above the surface, nothing in the world could ever compare to a view like this—hell, an _experience_ like this.

He wasn’t expecting to fall in love when he made the research trip of a lifetime, but… here he is. Sitting at a table and eating rehydrated eggs forty thousand feet below the surface of the ocean, bumping knees with a creature who walks and talks like a human but doesn’t need to breathe and knows things about the world that he, by all rights, should not.

But he wouldn’t change a thing.

Plus, it’s not like he hasn’t also achieved his original goal; his lab and storage are full of specimens that he’s collected in his time down here, and the ship’s computer loaded with diagnostic scans and notes and audio clips that Dean has made during his explorations. He has more than enough data to keep him occupied for the rest of his scientific career, and to make a name for himself in a big way.

He might keep the discovery of a huge, winged, Colossal Abyssal Specimen to himself, though. That one is going to be his and Cas’s little secret.

It feels strange, properly storing everything for the ascent and preparing himself to say goodbye to this strange little world he’s found himself in for the last two weeks. They make one final dive together—they don’t really have the time to spare, but it feels so _wrong_ to leave here without it, and without paying his last respects to this incredible, surreal place. He has his sub, he could come back down here, but the last thing Dean wants is for it to be torn apart by some sections of the scientific community. No, better for its location to stay a secret.

He lets himself take it all in one last time as Cas changes back into his magnificent underwater form. All huge wings and luminescent markings, he looks at home here, and for a moment, Dean wonders if they’re making the right decision. He cares for Cas, wants to make a home with him above the surface, but this has been Cas’s home for more time than either of them knows. He _belongs_ here, and it only becomes more evident as he swoops across the cavern with powerful strokes of his wings, weaving amongst groups of jellyfish and clouds of blue krill.

And then Cas turns and dives back down to Dean, stopping just short of him with a flare of wings that buffets the water between them. Between one breath and the next, Cas is human again, and there’s no hesitation as he swims up to where Dean is floating in the water.

He takes Dean’s hand in his, and he smiles, and Dean knows, in that moment, that Cas has made peace with his decision to leave.

His heart double-beats with joy against his ribs, and he grins back at Cas through the plastic of his mask. Hand in hand, they swim back to the ship together.

~

“Ready?” Dean asks, looking over to where Cas is strapped in to one of the emergency chairs a few feet behind him. Being a one man sub, they’ve had to make some alterations and some allowances, but it’s worked out in the end.

“I’m ready,” Cas says with a firm nod, although the way his fingers curl tightly around the straps of his harness suggest that he might be a little more nervous than he’s letting on. Dean crosses over to where he’s sitting and bends down to kiss him, slow and sure. When he pulls away, there’s a smile on Cas’s lips, and his death grip on the straps has eased.

“Everything’s gonna be okay, Cas,” Dean tells him, and he feels it in his heart. Him and Cas? They’re gonna be just fine, even outside of here, up above the surface in Dean’s world.

Cas’s smile widens. “I know,” he says, and it’s with such quiet confidence that it threatens to overwhelm him. Cas trusts him—enough to leave his home, enough to want to venture up to the surface and make a life together.

It’s more than a little surreal, and he holds onto that thought as he straps himself into his own chair and starts making the safety checks and preparations for their journey back to the surface. “Here we go,” he says, partly to Cas but also partly to himself. The ship’s motors—the ones with serious power, not just the baby ones he’s been using for the last two weeks—engage with a rumble, and out of the corner of his eye, Dean sees Cas tense up.

As soon as they make eye contact, though, Dean sees his tension ease just a little, and Cas gives him a nervous smile. _Fuck_ , Dean is so awed that Cas likes him enough to make this decision.

The computer counts down— _three, two, one_ —and then they’re moving up towards the roof of the cavern, and the great blue expanse of Cas’s home is stretching out below them and getting smaller and smaller. The galaxy of glow-worms above them grows closer as they near the ceiling, still just as dazzling as it had been the first time he’d seen it. The passage that leads to the surface is right above them now, and Dean reaches back with one hand towards Cas.

Fingers interlace with his, and Dean squeezes Cas’s hand, mouthing his own silent goodbye to the cavern as the ship’s computer navigates them up, up, into the mouth of the passageway.

He gets one last glimpse of the cavern, and then for the first time in what feels like forever, everything goes dark once more.

~

The journey to the surface is faster than it had been to descend, although the beginning of Dean’s expedition feels like it was a lifetime ago now. This time, he lets Verne navigate through the passage, now that he knows where it leads and he can trust the AI’s judgement. He and Cas stay strapped into their seats just in case (because Sam would kill him if something happened to him just because he wasn’t wearing his seatbelt) but pass the time by just talking about anything and everything, until—

The passage opens up, and suddenly they’re back in the known ocean, at the bottom of the Mariana Trench.

With a crackle of static, the radio comes to life.

“—to Winchester, come in Winchester. Control to Winchester, come in Winchester.”

Dean can’t help his grin at the sound of Charlie’s voice. He’s made it. He’s almost home.

“You’ve reached Winchester,” he says, pressing down on his comms button and unable to keep the joyful amusement out of his voice. “Over.”

There’s a pause, and then— “Dean!” Charlie shouts across the radio. “Holy shit, you’re alive! And you left it right up until the very last day, you fucking jerk. You’d better be on your way back up right the fuck now.”

Dean laughs, loud and happy—fuck, he’s missed his best friend. “Yeah, Charlie, I’m on my way up now. Boy, do I have a lot to tell you—and a surprise that is gonna blow your mind just a little bit.”

When he looks back, Cas has his eyebrows raised, a smile curling the edges of his lips. Dean winks at him—he gets the strangest feeling that Cas is going to get along with Sam and Charlie just fine. “I’ll see you in a few hours, okay Charles?”

“Sounds good, Dean. We’ll be waiting, and I can’t wait to hear all about it. Over and out.”

Dean sits back in his chair and runs his hands through his hair, blowing out a long breath. He feels fucking giddy, and he can’t _wait_ for Cas to meet his family. “So, that was Charlie,” he tells Cas with a grin. “She’s the crazy one I told you about. You’re gonna love her, and Sam, too. I promise.”

It takes them another two hours to ascend to the surface, and Dean watches the intrigue on Cas’s face as they pass by whales and fish and the water gradually gets lighter and lighter, more and more blue. They can see the sunlight now, and they’re so, _so_ close—

And just like that, the Jules Verne surfaces, popping up in the middle of the Philippine Sea like a giant metal cork. Half of Dean’s front window is water, and the other is bright blue sky.

Behind him, Cas gasps quietly.

“What do you think, Cas?” Dean asks as he unbuckles himself and stands up from his chair. Cas’s eyes are wide, and he’s not moving, just looking out at the sea and the sky and the white clouds gently scudding along above.

“It’s beautiful,” he breathes, and Dean smiles as he unfastens Cas’s buckles for him and leans in to press a quick kiss to his lips.

“Just wait until you see the rest of it,” he says, and fuck, he can’t remember the last time he was this _happy_ , this _excited_.

Before he can step away, Cas reaches up and curls his fingers into the front of Dean’s shirt, pulling him back in for a kiss that’s more heated, more desperate, fueled by emotions that Dean can’t put a finger on but can feel swirling around in the pit of his own stomach. When they pull back, they’re both breathless and smiling. “I can’t wait,” Cas says quietly.

It only takes a minute for Dean to disable the deep-sea safety measure and open the top hatch, and then it’s time. He goes first, climbing up the ladder hand over hand and pulling himself up onto the top of the sub. For the first time in three weeks, the breeze and the sun’s rays caress his face, and he laughs happily. A hundred feet away from the sub is Sam and Charlie’s ship, and he can see Sam on the deck, waving his arms in a greeting.

Dean lifts an arm to wave back, then looks back down into the ship, where Cas is standing at the bottom of the ladder and looking up at him. “You comin’ up?” he asks, and Cas starts to climb, slowly at first, and then with more confidence.

This was meant to be the expedition that skyrocketed Dean’s career, that _changed_ his _life_ —and it has, but not in a way that he could ever have anticipated.

He reaches down his hand, and helps Cas up into the sunlight.

**Notes for the Chapter:**

> Aghmi ealayh: fainted (Arabic)  
> Deficisti: fainted (Latin)  
> Bilmiyorum: I don't know (Turkish)  
> Olani hoath ol: I love you (unknown)
> 
> If you liked this, please leave a kudos and/or comment! They fuel my writing fire :)
> 
> You can find me on tumblr [here](http://saltnhalo.tumblr.com), and subscribe to me on ao3 [here](https://archiveofourown.org/users/saltnhalo) <3


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